Guppies are very easy to breed. In fact, they are also known as the millions fish because they breed faster than rabbits.
If you wish to keep as many of the fry as possible you need to separate mum from fry or she will eat them (as will all the other tank inhabitants). If they are in a well planted tank with plenty of places to hide, you can leave them to reproduce happily all together. Remember, you will soon run out of space if you keep all the fry all the time as they can drop anywhere from a few to a few hundred every 4 - 5 weeks